1. Field of the Invention
The present invention relates to an optical encoder.
2. Description of the Related Art
Optical encoders are used to acquire information on the motion of a moving object, such as the moving direction, position, velocity and the like. For example, a known optical encoder used together with a rotational object detects light passing through rotational slits that rotate together with the rotational object by means of a light receiving part to acquire information on the rotational motion of the rotational object.
In a conventional optical encoder, a light receiving part is arranged in alignment with rotational slits. Specifically, in order to increase the output level of the detection signal, the rotational slits and the light receiving part are arranged so that their edges are aligned with each other so as to allow the light going straight through the rotational slits to reach the light receiving part.
JP S62-007174A discloses a rotational angle detecting device in which the arrangement length of a light receiving element array is larger than the pitch of slit array formed on a rotary slit plate, in consideration of the fact that light emitted from a point light source reaches the light receiving element array while spreading. This related art is intended to prevent an insensitive area from being formed so as to improve detecting accuracy of the rotational angle.
In an optical encoder that is configured to detect light beams that have passed through a plurality of slits separately from one another, crosstalk may occur due to light beams passing through neighboring slits. FIG. 5 shows a positional relationship between a slit array and a light receiving part in an encoder according to the related art. A rotary plate 100 is formed with five slit arrays 101 to 105 spaced apart from each other. A light detecting unit 200 arranged to face the rotary plate 100 includes light receiving parts 201 to 205 corresponding to slit arrays 101 to 105, respectively.
The slit arrays 101 to 105 and corresponding light receiving parts 201 to 205 have common central axis lines A1 to A5, respectively, and extend in the same range. Accordingly, as indicated by the dashed lines in FIG. 5, the edges of the slit arrays 101 to 105 are aligned with the edges of the corresponding light receiving parts 201 to 205.
FIG. 6 shows an exemplary behavior of the light emitted from a light emitting part in an encoder according to the related art. As illustrated, the light emitted from a light emitting part 300 is not a perfect collimated beam (light directed parallel to the optical axis 310). Light L1 directed at an angle to the optical axis 310 passes through a rotational slit 102, but reaches a different light receiving part 201 which is adjacent to a light receiving part 202 corresponding to the rotational slit 102. Similarly, light L2 directed at an angle to the optical axis 310 passes through a rotational slit 104 and reaches a different, adjacent light receiving part 205, instead of the corresponding light receiving part 204.
In this way, if light having passed through a rotational slit different from the corresponding rotational slit reaches the light receiving part, the output signal from the light receiving part will be affected (which will be hereinafter simply referred to as “light crosstalk”). As a result, the detection accuracy of the encoder degrades.
Therefore, there is a need for an optical encoder which can prevent light crosstalk from occurring.